Plastic lenses are lightweight, highly tough and easy to be dyed. Properties particularly required for plastic lenses are: low specific gravity; high transparency; low yellowness; high refractive index and high Abbe number as optical properties; high heat resistance; high strength; and the like. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens.
Recently, many organic compounds having a sulfur atom to be used for providing a high refractive index and a high Abbe number have been reported. Among such compounds, polyepisulfide compounds having a sulfur atom are known to provide a good balance between the refractive index and the Abbe number (Patent Document 1).
In addition, optical materials in which thiourethane was introduced in a polyepisulfide compound in order to improve the strength have been reported (Patent Documents 2 and 3).
However, when thiourethane is introduced, it causes reduction in heat resistance, generation of odor at the time of cutting work, occurrence of white turbidity of lenses and occurrence of uneven polymerization called striae, and for this reason, techniques in which a composition ratio is limited or a viscosity is limited have been reported (Patent Documents 4-6).
Further, there is a report of an optical material, in which sulfur was introduced in a polyepisulfide compound in order to maintain the refractive index and thiourethane was introduced in the compound in order to improve impact resistance (Patent Document 7).
However, optical materials obtained by introducing sulfur and thiourethane in a polyepisulfide compound have problems of foam formation, heat generation, etc. at the time of the production. For this reason, there is a report of a method for producing an optical material, in which a product obtained by preliminarily polymerizing an episulfide compound with a sulfur atom is reacted with a product obtained by preliminarily polymerizing a compound having an isocyanate group with a compound having a mercapto group, for the purpose of controlling the above-described problems (Patent Document 8).
Further, a method for prescribing a formulation method for the purpose of suppressing white turbidity and uneven polymerization called striae of lenses obtained has been reported (Patent Document 9).